The present invention relates to the preparation of matrices by the polymerization of macromers. In another aspect, the invention relates to the use of such matrices for such purposes as cell immobilization, tissue adherence, and controlled drug delivery.
Matrices are polymeric networks characterized by insolubility in water. One type of polymeric matrix is a hydrogel, which can be defined as a water-containing polymeric network. The polymers used to prepare hydrogels can be based on a variety of monomer types, such as those based on methacrylic and acrylic ester monomers, acrylamide (methacrylamide) monomers, and N-vinyl-2-pyrrolidone. To form the gel, these monomer classes are typically crosslinked with such crosslinking agents as ethylene dimethacrylate, N,Nxe2x80x2-methylenebisacrylamide, methylenebis(4-phenyl isocyanate), ethylene dimethacrylate, divinylbenzene, and allyl methacrylate.
Another type of polymeric network can be formed from more hydrophobic monomers and/or macromers. Matrices formed from these materials generally exclude water. Polymers used to prepare hydrophobic matrices can be based on a variety of monomer types such as alkyl acrylates and methacrylates, and polyester-forming monomers such as xcex5-caprolactone and lactide. When formulated for use in an aqueous environment, these materials do not need to be crosslinked, but they can be crosslinked with standard agents such as divinyl benzene. Hydrophobic matrices can also be formed from reactions of macromers bearing the appropriate reactive groups such as the reaction of diisocyanate macromers with dihydroxy macromers, and the reaction of diepoxy-containing macromers with dianhydride or diamine-containing macromers.
Although there exist a variety of methods for producing polymeric networks, when these networks are intended to be created in the presence of viable tissue, and/or to contain a bioactive compound, the number of acceptable methods of producing polymeric networks is extremely limited.
It is nevertheless desirable to form both hydrogel and non-hydrogel polymeric matrices in the presence of viable tissue or bioactive agents for the purposes of drug delivery, cellular immune isolation, prevention of post-surgical adhesions, tissue repair, and the like. These polymeric matrices can be divided into two categories: biodegradable or bioresorbable polymer networks and biostable polymer networks.
Biodegradable polymeric matrices have been previously suggested for a variety of purposes, including controlled release carriers, adhesives and sealers. When used as controlled release carriers, for instance, polymeric matrices can contain and release drugs or other therapeutic agents over time. Such matrices can be formed, for instance, by a number of different processes, including solvent casting hydrophobic polymers. Solvent casting, however, typically involves the use of organic solvents and/or high temperatures which can be detrimental to the activity of biological materials and can complicate production methods. Solvent casting of polymers out of solution also results in the formation of uncrosslinked matrices. Such matrices have less structure than crosslinked matrices and it is more difficult to control the release of bioactive agents from such matrices. Yet another process, which involves the polymerization of monomers in or around the desired materials, suffers from cytotoxicity of monomers, oxygen inhibition and heat of polymerization complications.
Another process used in the past to prepare biodegradable and biostable hydrogels involves the polymerization of preformed macromers using low molecular weight initiators. This process involves a number of drawbacks as well, however, including toxicity, efficacy, and solubility considerations. For instance, when using a macromer solution containing a low molecular weight soluble initiator to encapsulate viable cellular material, the initiator can penetrate the cellular membrane and diffuse into the cells. The presence of the initiator may involve some toxic consequence to the cells. When activated, however, these initiators produce free radicals having distinct cytotoxic potential. Other drawbacks arise if the initiator is able to diffuse out of the formed matrix, thereby producing toxicity and other issues. Such initiators also tend to aggregate in aqueous solution, causing efficiency and reproducibility problems. Finally, in view of the limited efficiency of many initiators for initiating the necessary radical chain polymerization, it is often necessary to add one or more monomeric polymerization xe2x80x9cacceleratorsxe2x80x9d to the polymerization mixture. Such accelerators tend to be small molecules capable of penetrating the cellular membrane, and often raise cytotoxic or carcinogenic concerns.
U.S. Pat. Nos. 5,410,016 (Hubbell, et al.) and 5,529,914 (Hubbell, et.al.) for instance, relate to hydrogels prepared from biodegradable and biostable polymerizable macromers. The hydrogels are prepared from these polymerizable macromers by the use of soluble, low molecular weight initiators. Such initiators can be combined with the macromers, and irradiated in the presence of cells, in order to form a gel that encapsulates the cells. A considerable number of similar and related patents have arisen over recent years. See, for instance, U.S. Pat. Nos. 5,232,984; 5,380,536; 5,573,934; 5,612,050; 5,837,747; 5,846,530; and 5,858,746.
Hydrogels often suffer from similar or other drawbacks in use as biological adhesives or sealants, e.g., for use as tissue adhesives, endovascular paving, prevention of post-surgical adhesions, etc. In each of the applications, the hydrogel matrix must generally xe2x80x9cadherexe2x80x9d to one or more tissue surfaces. Current methods rely upon physical xe2x80x9cadhesionxe2x80x9d or the tendency of hydrogels to xe2x80x9cstickxe2x80x9d to a surface. A superior adhesive would provide both physical and chemical adhesion to surfaces utilizing the same physical characteristics as current hydrogel adhesives, but also providing chemical, covalent coupling of the matrix material to the tissue surface. Covalent bonds are generally much stronger than physical adhesive forces, such as hydrogen bonding and van der Waals forces.
As described above, when various techniques are used to form polymeric matrices via photoinitiation of macromers, the photoinitiators utilized tend to be low molecular weight. Polymeric photoinitiators have been described as well, although for applications and systems quite distinct from those described above. See, for instance, xe2x80x9cRadical Polymerizationxe2x80x9d, C. H. Bamford, pp. 940-957 in Kroschwitz, ed., Concise Encyclopedia of Polymer Science and Engineering, 1990. In the subsection entitled xe2x80x9cPhotosensitized Initiation: Polymeric Photosensitizers and Photoinitiatorsxe2x80x9d, the author states that xe2x80x9c[p]olymeric photosensitizers and photoinitiators have been described. Many of these are polymers based on benzophenone, e.g., poly(p-divinylbenzophenone) (DVBP). Such rigid polymers are reported to be effective sensitizers since hydrogen abstraction from the backbone by excited benzophenone is less likely.xe2x80x9d
U.S. Pat. No. 4,315,998 (Neckers) describes polymer-bound photosensitizing catalysts for use in the heterogeneous catalysis of photosensitized chemical reactions such as photo-oxidation, photodimerization, and photocyclo addition reactions. The polymer-bound photosensitizing catalysts are insoluble in water and common organic solvents, and therefore can be readily separated from the reaction medium and reaction products by simple filtration.
What is clearly needed are macromers and macromer systems that avoid the problems associated with conventional polymeric matrices, and in particular, those drawbacks that arise when polymeric matrices are formed in the presence of viable tissue or bioactive agents.
The present invention provides a method of improving the performance, within a host tissue site, of an implanted article (e.g., implanted tissue or implanted prosthetic device providing a porous surface), by promoting the growth of continuous tissue between host tissue and the implant. An implanted article of this invention can be provided, for instance, in the form of cultured tissue and/or native tissue (e.g., transplanted tissue), or can be fabricated from polymeric and/or metallic materials. The method employs a crosslinkable macromer system adapted to form an interface between the article and the host tissue, in order to facilitate (e.g., promote and/or permit) tissue integration by the body between the implant and the tissue site, e.g., into and through the crosslinked macromer and into the porosity of a porous device surface.
Applicant""s parent application Ser. No. 09/121,248 (now U.S. Pat. No. 6,007,833) provides examples of preferred macromer systems useful in the method of the present invention. The parent application teaches the use of such systems for various applications, including cellular encapsulation, adhesives and sealants, barriers, controlled release carriers, tissue replacement/scaffolding, wound dressings, and in situ device formation. For use in tissue replacement, for instance, the parent application teaches the placement of a macromer system in a mold or cavities in a device.
Polymeric matrices of this invention (as formed by crosslinking the macromer system) can be used, for instance, to improve the tissue response to implanted medical devices. Examples of improved tissue response include: 1) increased tissue ingrowth (e.g., into the pores of cementless hip implants), and, 2) decreased fibrosis (e.g., around breast implants and hernia repair meshes). In addition to tissue integration, the macromer system can provide a variety of advantages, including a reduction in both immediate and chronic adverse reactions to the device (e.g., thereby preventing the accumulation of fluid and/or undesirable cells) by virtue of the immediate and desirable space filling characteristics.
A macromer system of this invention can be provided between the implant and tissue site in any suitable manner, e.g., the system can be provided upon the surface of the implant before, during and/or after placement of the implant in the tissue site. Similarly, the macromer system(s) can be polymerized to form a matrix at any suitable point, e.g., a system can be coated on an implant, and/or delivered to the tissue site, and crosslinked either before, during and/or after positioning the implant in or upon the tissue site. For example, a system can be provided and crosslinked on the surface of a prosthetic device, e.g., by either the manufacturer or surgeon, and implanted into the tissue site. As another example, a macromer system can be applied to the tissue site itself, polymerized in situ (e.g., by the application of illumination either directly to the system or through translucent tissue), and an implant article (with or without a macromer system or matrix on its surface) positioned on or in the site.
The resulting combination of implant and matrix within the tissue site, permits the formation of continuous tissue growth, over time, through the matrix and between the implant and tissue site. Such growth can be unidirectional (e.g., originating from the tissue site toward the implant) and/or bidirectional (e.g., originating from both the tissue site and from an implanted tissue).
A system, or resultant matrix, can be applied with or without additional components, such as growth factors, morphogenic factors, or DNA. Both degradable and non-degradable macromer systems are useful, but matrices adapted to degrade with desired kinetics, are preferred.
In another aspect, the present invention provides a combination comprising an implant article, such as a tissue implant or prosthetic device having one or more porous surfaces, and a matrix formed of a crosslinked macromer system as described herein. In one embodiment, the article and uncrosslinked macromer system are initially combined (e.g., the system coated on the article) outside the body, thereby permitting the system to be crosslinked prior to, during and/or following placement of the implant within the body site. In an alternative embodiment, the combination is provided by positioning an article within and/or in apposition to the body site, and delivering the macromer system to the body site (before, during and/or after placement of the article itself), where it is crosslinked as a sufficiently stable coating or other suitable interface in its position between the article and the tissue site.
For instance, a macromer system in liquid (e.g., substantially flowable) form can be applied to a tissue defect, before, during or after which a tissue implant or preformed device is pressed into the defect. The system can be provided, for instance, to occupy a space of between about 0.1 mm to about 10 mm between the implant and the tissue site. The system is illuminated in order to activate the initiator groups and thus polymerize the macromer system into a solid matrix. The resultant combination of tissue implant or preformed device, together with crosslinked macromer system, completely and conformally fills the defect such that no gaps remain for the accumulation of undesirable fluids or cells.
As described herein, the macromer system can thus be used in the manner of a xe2x80x9cgroutxe2x80x9d, for instance, to fill the spaces between a tissue implant or preformed device (itself either tissue-based or non-tissue based) and adjacent tissue. Current tissue implants include, for instance, both those obtained as transplants (e.g., autografts, allografts or xenografts) and those provided by tissue engineering. Current tissue engineering products often consist of cultured tissues that are implanted into tissue defects. Such products do not typically conform well to adjacent native tissue, however, thus leaving spaces into which undesirable fluids and cells can accumulate and produce adverse tissue responses. For example, when cultured cartilage is implanted into cartilage defects, synovial fluid and macrophages can enter the unfilled space and lead to fibrous tissue formation, which prevents integration of the implanted cartilage with the native cartilage. Other cultured tissues that are implanted into tissue defects, and that would benefit from the present macromer system applied as a grout include, but are not limited to, skin, bone, ligaments, blood vessels, and heart valves.
Implants, e.g., prosthetic devices, useful in a combination or method of this invention include those in which tissue integration is desired, and that themselves provide (or can be provided with) a sufficiently porous surface that permits or facilitates tissue integration once positioned in vivo. Examples of suitable porous prosthetic devices include, but are not limited to; joint implants (e.g., for hip or knee reconstruction), dental implants, soft tissue cosmetic prostheses (e.g., breast implants), wound dressings, vascular prostheses (e.g., vascular grafts and stents), and ophthalmic prostheses (e.g., intracorneal lenses). The macromer system of this invention, in turn, can be used in any suitable manner, e.g., to coat and/or fill voids within or upon the surface of the prosthetic device.
Such devices preferably are themselves formed of or otherwise provide (or can be provided with) a surface having sufficient porosity to permit tissue ingrowth in vivo. As used herein, the word xe2x80x9cporousxe2x80x9d, and inflections thereof, will refer to one or more portions of the device surface that are designed for direct or indirect contact with the surrounding natural tissue, sufficient to permit tissue integration into the porosity thereof. Porous surfaces can be provided in a variety of ways, e.g., as sintered particles on a surface, such as titanium particles on the surface of cementless hip implants, as are available from a variety of orthopedic companies. Porous surfaces can also be provided in the form of cavities that remain from mixing salt crystals with silicone rubber oligomers, then solidifying (vulcanizing) the silicone rubber, and finally dissolving the salt crystals (as currently done for a variety of breast implants). Yet other porous surfaces can be fabricated from fibrous materials or produced as porous sponges via solvent casting and particulate leaching, phase separation, or gas foaming (see, e.g., B. S. Kim and D. J. Mooney, Development of Biocompatible Synthetic Extracellular Matricies for Tissue Engineering TIBTECH 16:224-230 (1998)).
Such porous surfaces typically provide a three-dimensional structure of spaces into which tissue can grow and mature. Porous regions of an implantable device preferably have a high pore density, in that the pores themselves occupy a greater relative volume than the material forming and separating those pores. Desirably, the pores have interconnected passages that allow direct contact between the tissue growing in adjacent pores. The minimum average pore size is preferably sufficient to accommodate capillaries (e.g., of about 5 micron diameter) and the maximum average pore size is about 1 mm. Preferable pore sizes will vary from tissue to tissue and typically range from about 20 microns to about 600 microns, and more preferably from about 50 microns to about 400 microns.
A crosslinkable macromer system useful in the present invention comprises two or more polymer-pendent polymerizable groups and one or more initiators, preferably in the form of polymer-pendent initiator groups. In a preferred embodiment, the polymerizable groups and the initiator group(s) are pendent on the same polymeric backbone. In an alternative preferred embodiment, the polymerizable groups and initiator group(s) are pendent on different polymeric backbones.
In the first embodiment, the macromer system comprises a polymeric backbone to which are covalently bonded both the polymerizable groups and initiator group(s). Pendent initiator groups can be provided by bonding the groups to the backbone at any suitable time, e.g., either prior to the formation of the macromer (for instance, to monomers used to prepare the macromer), or to the fully formed macromer itself. The macromer system itself will typically comprise but a small percentage of macromers bearing both initiator groups and polymerizable groups. The majority of macromers will provide only pendent polymerizable groups, since the initiator groups are typically sufficient if present at far less than 1:1 stoichiometric ratio with macromer molecules.
In an alternative preferred embodiment, the macromer system comprises both polymerizable macromers, generally without pendent initiator groups, in combination with a polymeric initiator. In either embodiment, the initiator will be referred to herein as a xe2x80x9cpolymeric initiatorxe2x80x9d, by virtue of the attachment of such initiator groups to a polymeric backbone. Yet another embodiment of this invention includes the macromer system having free (non-polymer bound) initiator molecules.
Macromer systems of the present invention, employing polymeric initiators, provide a number of unexpected advantages over the use of polymerizable macromers and separate, low molecular weight initiators. Such systems, for instance, provide an optimal combination of such properties as nontoxicity, efficiency, and solubility. Solubility, for instance, can be improved by virtue of the ability to control the aqueous or organic solubility of the polymerizable macromer by controlling the backbone. Toxicity can also be improved, since the polymeric initiators of this invention typically cannot diffuse into cells in the course of immobilization.
In a preferred embodiment, the pendent initiator groups are selected from the group consisting of long-wave ultra violet (LWUV) light-activatable molecules such as; 4-benzoylbenzoic acid, [(9-oxo-2-thioxanthanyl)-oxy]acetic acid, 2-hydroxy thioxanthone, and vinyloxymethylbenzoin methyl ether; visible light activatable molecules; eosin Y, rose bengal, camphorquinone and erythrosin, and thermally activatable molecules; 4,4xe2x80x2 azobis(4-cyanopentanoic) acid and 2,2-azobis[2-(2-imidazolin-2-yl) propane] dihydrochloride. An important characteristic of the initiator group being the ability to be coupled to a preformed macromer containing polymerizable groups, or to be modified to form a monomer which can take part in the macromer synthesis, which is subsequently followed by the addition of polymerizable groups.
In such an embodiment, the pendent polymerizable groups are preferably selected from the group consisting of pendent vinyl groups, acrylate groups, methacrylate groups, ethacrylate groups, 2-phenyl acrylate groups, acrylamide groups, methacrylamide groups, itaconate groups, and styrene groups.
In a further preferred embodiment, the polymeric backbone is selected from the group consisting of synthetic macromers, such as polyvinylpyrrolidone (PVP), polyethylene oxide (PEO), and polyethylene glycol (PEG); derivatizable naturally occurring polymers such as cellulose; polysaccharides, such as hyaluronic acid, dextran, and heparin; and proteins, such as collagen, gelatin, and albumin.
The macromers of the present invention can be used in a variety of applications, including controlled drug release, the preparation of tissue adhesives and sealants, the immobilization of cells, and the preparation of three-dimensional bodies for implants. In one aspect, for instance, the invention provides a method for immobilizing cells, the method comprising the steps of combining a polymeric initiator of the present invention with one or more polymerizable macromers and in the presence of cells, under conditions suitable to polymerize the macromer in a manner that immobilizes the cells.
As used herein the following words and terms shall have the meaning ascribed below:
xe2x80x9cmacromer systemxe2x80x9d shall refer to a polymerizable polymer system comprising one or more polymers providing pendent polymerizable and initiator groups. Groups can be present either on the same or different polymeric backbones, e.g., on either a polymerizable macromer or a non-polymerizable polymeric backbone;
xe2x80x9cpolymerizable macromerxe2x80x9d shall refer to a polymeric backbone bearing two or more polymerizable (e.g., vinyl) groups;
xe2x80x9cinitiator groupxe2x80x9d shall refer to a chemical group capable of initiating a free radical reaction, present as either a pendent group on a polymerizable macromer or pendent on a separate, non-polymerizable polymer backbone; and
xe2x80x9cpolymeric initiatorxe2x80x9d shall refer to a polymeric backbone (polymerizable or non-polymerizable) comprising one or more initiator groups and optionally containing one or more other thermochemically reactive groups or affinity groups.
The polymeric backbone of this invention can be either synthetic or naturally-occurring, and includes a number of macromers previously described as useful for the preparation of polymeric matrices. Generally, the backbone is one that is soluble, or nearly soluble, in aqueous solutions such as water, or water with added organic solvent (e.g., dimethylsulfoxide) or can be rendered soluble using an appropriate solvent or combination of solvents. Alternatively, the polymeric backbone can be a material which is a liquid under ambient physiological conditions. Backbones for use in preparing biodegradable gels are preferably hydrolyzable under in vivo conditions.
In general, the polymeric backbones of this invention can be divided into two categories: biodegradable or bioresorbable, and biostable reagents. These can be further divided into reagents which form hydrophilic, hydrogel matricies and reagents which form non-hydrogel matricies.
Bioresorbable hydrogel-forming backbones are generally naturally occurring polymers such as polysaccharides, examples of which include, but are not limited to, hyaluronic acid (HA), starch, dextran, heparin, chondroitin sulfate, dermatan sulfate, heparan sulfate, keratan sulfate, dextran sulfate, pentosan polysulfate, and chitosan; and proteins (and other polyamino acids), examples of which include but are not limited to gelatin, collagen, fibronectin, laminin, albumin, elastin, and active peptide domains thereof. Matrices formed from these materials degrade under physiological conditions, generally via enzyme-mediated hydrolysis.
Hyaluronic acid, when derivatized with polymerizable groups in the manner described herein, provides a variety of advantages and benefits not heretofore known or achievable. Hyaluronic acid has conventionally been derivatized in aqueous conditions (see, e.g., Soon-Shiong (U.S. Pat. No. 5,837,747), Sakurai (U.S. Pat. No. 4,716,224), and Matsuda (U.S. Pat. No. 5,763,504). Occasional references have described the ability to derivatize hyaluronic acid under organic conditions (see, e.g., Della Valle, U.S. Pat. No. 5,676,964). In most, if not all, such approaches, however, the reaction mixtures are either suspensions, as opposed to true solutions, or the hyaluronic acid is itself pre-reacted (typically in a predominantly aqueous mixture) to enhance its solubility in organic solvents.
Applicants have discovered the manner in which hyaluronic acid, as well as other polysaccharides and polyamino acids (such as collagen) can be effectively derivatized in organic, polar, anhydrous solvents and solvent combinations. A particularly preferred solvent is formamide, and combinations of other solvents therewith. Functionally, the solvent or solvent system is one in which the polymer is sufficiently soluble and that permits its derivatization to the desired extent. The ability to derivatize such polymers in the manner of this invention provides a variety of unexpected benefits, and an optimal combination of such properties as preparation cost, controllability, and yield.
As exemplifed below, for instance, hyaluronic acid is reacted in formamide (and TEA, for pH control) with a reactive moiety in the form of glycidyl acrylate in order to derivatize the hyaluronic acid molecules with acrylate groups. The number and/or density of acrylate groups can be controlled using the present method, e.g., by controlling the relative concentration of reactive moiety to saccharide group content.
Bioresorbable matrix-forming backbones are generally synthetic polymers prepared via condensation polymerization of one or more monomers. Matrix-forming polymers of this type include polylactide (PLA), polyglycolide (PGA), polycaprolactone (PCL), as well as copolymers of these materials, polyanhydrides, and polyortho esters.
Biostable hydrogel matrix-forming backbones are generally synthetic or naturally occurring polymers which are soluble in water, matrices of which are hydrogels or water-containing gels. Examples of this type of backbone include polyvinylpyrrolidone (PVP), polyethylene glycol (PEG), polyacrylamide (PAA), polyvinyl alcohol (PVA), and the like.
Biostable matrix-forming backbones are generally synthetic polymers formed from hydrophobic monomers such as methyl methacrylate, butyl methacrylate, dimethyl siloxanes, and the like. These backbone materials generally do not possess significant water solubility but can be formulated as neat liquids which form strong matrices upon activation. It is also possible to synthesize backbone polymers which contain both hydrophilic and hydrophobic monomers.
Polymeric backbones of polymerizable macromers can optionally provide a number of desirable functions or attributes, e.g., as described in the above-captioned Hubbell patents, the disclosures of which are incorporated herein by reference. Backbones can be provided with water soluble regions, biodegradable regions, hydrophobic regions, as well as polymerizable regions.
As used herein, the term xe2x80x9cpolymerizable groupxe2x80x9d will generally refer to a group that is polymerizable by initiation by free radical generation, most preferably by photoinitiators activated by visible or long wavelength ultraviolet radiation. Preferred polymerizable groups include acrylates, methacrylates, ethacrylates, itaconates, acrylamides, methacrylamide, and styrene.
Typically, polymerizable groups are incorporated into a macromer subsequent to the initial macromer formation using standard thermochemical reactions. Thus, for example, polymerizable groups can be added to collagen via reaction of amine containing lysine residues with acryloyl chloride or glycidyl acrylate. These reactions result in collagen containing pendent polymerizable moieties. Similarly, when synthesizing a macromer for use as described in the present invention, monomers containing reactive groups can be incorporated into the synthetic scheme. For example, hydroxyethylmethacrylate (HEMA) or aminopropylmethacrylamide (APMA) can be copolymerized with N-vinylpyrrolidone or acrylamide yielding a water-soluble polymer with pendent hydroxyl or amine groups. These pendent groups can subsequently be reacted with acryloyl chloride or glycidyl acrylate to form water-soluble polymers with pendent polymerizable groups.
Initiator groups useful in the system of the present invention include those that can be used to initiate, by free radical generation, polymerization of the macromers to a desired extent and within a desired time frame. Crosslinking and polymerization are generally initiated among macromers by a light-activated free-radical polymerization initiator. Preferred initiators for long wave UV and visible light initiation include ethyl eosin, 2,2-dimethoxy-2-phenyl acetophenone, other acetophenone derivatives, thioxanthone, benzophenone, and camphorquinone.
Preferred polymeric initiators are photosensitive molecules which capture light energy and initiate polymerization of the macromers. Other preferred polymeric initiators are thermosensitive molecules which capture thermal energy and initiate polymerization of the macromers.
Photoinitiation of the free radical polymerization of macromers of the present invention will generally occur by one of three mechanisms. The first mechanism involves a homolytic alpha cleavage reaction between a carbonyl group and an adjacent carbon atom. This type of reaction is generally referred to as a Norrish type I reaction. Examples of molecules exhibiting Norrish type I reactivity and useful in a polymeric initiating system include derivatives of benzoin ether and acetophenone.
The second mechanism involves a hydrogen abstraction reaction, either intra- or intermolecular. This initiation system can be used without additional energy transfer acceptor molecules and utilizing nonspecific hydrogen abstraction, but is more commonly used with an energy transfer acceptor, typically a tertiary amine, which results in the formation of both aminoalkyl radicals and ketyl radicals. Examples of molecules exhibiting hydrogen abstraction reactivity and useful in a polymeric initiating system, include analogs of benzophenone, thioxanthone, and camphorquinone.
When using a polymeric initiator of the hydrogen abstraction variety, pendent tertiary amine groups can be incorporated into the polymeric backbone of the macromer. This will insure that all free radicals formed are polymer-bound.
The third mechanism involves photosensitization reactions utilizing photoreducible or photo-oxidizable dyes. In most instances, photoreducible dyes are used in conjunction with a reductant, typically, a tertiary amine. The reductant intercepts the induced triplet producing the radical anion of the dye and the radical cation of the reductant. Examples of molecules exhibiting photosensitization reactivity and useful in a polymeric initiating system include eosin Y, rose bengal, and erythrosin. Reductants can be incorporated into the polymer backbone, thereby assuring that all free radicals will be polymer-bound.
Thermally reactive polymeric initiators are also useful for the polymerization of macromers. Examples of thermally reactive initiators usable in a polymeric initiating system include 4,4xe2x80x2 azobis(4-cyanopentanoic acid) and analogs of benzoyl peroxide.
A surprisingly beneficial effect of the use of polymeric initiators to polymerize macromers is the increased efficiency of polymerization exhibited by these polymeric initiators as compared to their low molecular weight counterparts. This increased efficiency is seen in all three photoinitiation mechanisms useful for the polymerization of macromers.
Polymeric initiation of monomer solutions has been investigated for its application in the field of UV-curable coatings for industrial uses, c.f. U.S. Pat. No. 4,315,998 (Neckers) and PCT Application, International Publication No. WO 97/24376 (Kuester, et.al.) but there have been no reports of the adaptation of the use of polymeric initiators for the polymerization of macromers in the presence of biologic material or for the creation of drug-releasing matrices.
High efficiency of initiation is particularly important in systems such as these. It is generally desirable, when forming polymeric matrices in the presence of biologic or bioactive materials, to minimize the exposure time of the material to the energy source used to initiate polymerization. It is therefore imperative that the initiation system utilized possess optimum initiation efficiency.
When matrix strength or durability are required for a particular application, high efficiency is again a necessary characteristic of an initiation system. When a matrix-forming system is initiated, the free radical polymerization of the system is propagated until gelation and vitrification of the polymerizing system render the diffusion of the elements of the matrix-forming system too difficult. Therefore, the higher the efficiency of the initiation system, the more complete the polymerization resulting in the formation of stronger, more durable matrices. The polymeric initiation systems described in this invention provide a higher degree of efficiency, with or without the use of accelerants, than is attainable using nonpolymer-bound, low molecular weight initiators.
Another beneficial effect is realized when the initiating groups on the polymeric initiators consist of groups exhibiting hydrogen abstraction reactivity, i.e., the ability to abstract hydrogens intermolecularly. The beneficial effect is important when macromer systems containing these initiators are used as tissue adhesives, endovascular paving, formation of barriers to prevent post-surgical adhesions, or any application involving the xe2x80x9cadhesionxe2x80x9d of the matrix to one or more surfaces. Since initiators exhibiting this type of reactivity can abstract hydrogens from adjacent molecules, when a macromer system containing polymeric initiators of this type is applied to a substrate, photoactivation of the system causes the abstraction of hydrogens from the substrate by the initiators, thus forming a free radical on the substrate and a free radical on the initiator. This diradical can subsequently collapse forming a covalent bond between the macromer system and the substrate.
Other initiator groups on the same macromer initiate free radical reactions with other macromers resulting in the formation of a crosslinked matrix covalently bound to the surface. Initiator groups exhibiting this type of reactivity include analogs of benzophenone and thioxanthone. As can be readily understood, only polymeric initiators are capable of accomplishing this adhesion of the matrix to a surface, low molecular weight analogs of these initiators cannot produce this phenomenon.
Optionally, the use of initiators for initiating the necessary radical chain polymerization, can include the addition of one or more monomeric polymerization accelerants to the polymerization mixture, the accelerants serving to enhance the efficiency of polymerization. Polymerization accelerants useful in the present invention are typically monomers, which improve the reactivity of the macromer systems. Polymerization accelerants that have found particular utility for this application are N-vinyl compounds, particularly N-vinyl pyrrolidone and N-vinyl caprolactam. Such accelerants can be used, for instance, at a concentration of between about 0.01% and about 5%, and preferably between about 0.05% and about 0.5%, by weight, based on the volume of the macromer system.
In another embodiment, the polymeric initiator comprises a polymeric backbone with pendent initiator groups and pendent reactive or affinity groups. These reactive or affinity groups enable the polymeric initiator to bind to target groups on surfaces of interest. This allows the polymeric initiator to bind to the surface of interest. In this manner, interfacial polymerization of macromers can be accomplished. A solution of polymeric initiator-containing pendent reactive or affinity groups is applied to a surface with target sites. The reactive or affinity groups on the polymeric initiator react with the sites on the surface causing the polymeric initiator to bind to the surface. Excess polymeric initiator can then be washed away. A solution of a polymerizable macromer is then applied to the surface. When light energy in applied to the system, a free radical polymerization reaction is initiated only at the surface of interest. By varying the concentration of the polymerizable macromer and the illumination time, the thickness and crosslink density of the resulting matrix on the surface can be manipulated.
Generally, there are two methods by which an initiator group can be incorporated into a polymeric backbone. The first method involves the formation of a monomer which includes the initiator. This can be accomplished readily using standard chemical reactions. For example, the acid chloride analog of an initiator can be reacted with an amine-containing monomer, to form a monomer which contains the initiator.
The second method of incorporating initiator groups into a polymeric backbone involves coupling a reactive analog of the initiator with a preformed polymer. For example, an acid chloride analog of an initiator can be reacted with a polymer containing pendent amine groups forming a polymer bearing pendent initiator groups.
Macromer systems can be applied to a tissue site and/or implant article in any suitable manner, including by spraying, dipping, injecting or brushing the macromer system. Polymeric matrices prepared from macromer systems can be used in a variety of applications, including:
Cellular Encapsulation. The use of hydrogels to form micro- or macrocapsules containing cells and other tissue, is well documented in the literature. Applications include the treatment of diabetes, Parkinson""s disease, Alzheimer""s disease, ALS, chronic pain, and others. Descriptions of cellular encapsulation methods can be found throughout the patent and scientific literature. The use of the instant invention provides methods of encapsulating cells in two basic ways.
1) Bulk Polymerization
In this embodiment, cellular material is mixed in a solution of the macromer system and energy subsequently added to activate initiation of free radical polymerization. Prior to initiation, the solution containing the macromer system with suspended cellular material, can be placed in molds, shaped in particular geometric shapes, or placed inside a preformed membrane system, such as a hollow fiber. Upon illumination or other energy addition, the initiation of free radical polymerization causes the macromer system to gel, forming a cell-containing matrix in the desired shape. When formed into free-standing geometric shapes, the formulation of the macromer system can be designed to provide the desired degrees of durability and permselectivity to the subsequently formed matrix. When formed inside membrane structures, such as hollow fibers designed to provide the desired permselectivity, the macromer system can be formulated to provide the desired characteristics of the cell-suspending matrix, such as biocompatibility, etc.
2) Interfacial Polymerization
In this embodiment, a membrane is formed directly on the surface of the cellular material. A solution of polymerizable or non-polymerizable polymeric initiator-containing pendent affinity groups (e.g., positively charged groups) is mixed with the cellular material. The affinity groups bind to the sites on the surface of the cellular material. The excess polymeric initiator is subsequently washed away and the cellular material suspended in a solution of polymerizable macromer. Since initiator groups are present only at the surface of the cellular material, when light energy is applied, polymerization is initiated only at the surface:macromer interface. By manipulating the duration of illumination and macromer formulation, a polymeric matrix exhibiting the desired characteristics of thickness, durability, permselectivity, etc. is formed directly on the surface of the cellular material.
Adhesives and sealants. Polymeric matrix systems have also found extensive use as adhesives for tissue and other surfaces. For this application, a solution of a macromer system is applied to a surface to which adhesion is desired, another surface is contacted with this surface, and illumination is applied forming a surface-to-surface junction. If a temporary adhesive is desired, the macromer system can be composed of degradable macromers.
Barriers. Polymeric matrices can be used for the formation of barriers on surfaces for various applications. One such application is a barrier for the prevention of tissue adhesions following surgery. For this application, a macromer system in liquid form is applied to the surface of damaged tissue. The liquid is illuminated to polymerize the macromers. The polymeric matrix prevents other tissue from adhering to the damaged tissue. Both degradable and non-degradable macromer systems can be used for this purpose. As described above, both bulk polymerization and interfacial polymerization methods can be used to prepare surface coatings of this type.
Controlled Release Carriers. Polymeric matrices find wide application as controlled release vehicles. For this application, a solution of a macromer system and drug, protein, or other active substance is applied to a surface. The solution is illuminated to polymerize the macromers. The polymeric matrix contains the drug, when exposed to a physiological or other liquid-containing environment, the drug is slowly released into the environment. The release profile of the entrained drug can be manipulated by varying the formulation of the macromer system. Both degradable and non-degradable macromer systems can be utilized for this purpose. Likewise, both bulk and interfacial polymerization techniques can be used to prepare controlled drug-releasing surfaces. In an alternative embodiment, a drug or other active substance can be imbibed by a preformed matrix on a surface. The absorption and release characteristics of the matrix can be manipulated by varying the crosslink density, the hydrophobicity of the matrix, and the solvent used for imbibition.
Alternatively, drug-containing polymeric microspheres can be prepared using standard techniques. A wide range of drugs and bioactive materials can be delivered using the invention which include but are not limited to, antithrombogenic, anti-inflammatory, antimicrobial, antiproliferative, and anticancer agents, as well as growth factors, morphogenic proteins, and the like.
Tissue Replacement/Scaffolding. Polymeric matrices have found utility as three-dimensional scaffolding for hybrid tissues and organs. For this application, a macromer system in liquid form is applied to a tissue defect and subsequently illuminated to polymerize the macromers forming a matrix upon which ingrowing cells can migrate and organize into a functional tissue. In one embodiment, the macromer system additionally includes a growth factor which is slowly released and stimulates the ingrowth of desired cell types. In another embodiment, the macromers include pendent extracellular matrix peptides which can stimulate the ingrowth of desired cell types. A third embodiment would include both of the above features. An alternative embodiment includes cells included in the matrix with or without additional growth factor. The scaffolding can be generated in vitro by placing the liquid macromer system in a mold or cavities in a device, or can be generated in vivo by applying the liquid macromer system to a tissue defect. Both degradable and non-degradable macromer systems could be used for this application, but degradable matrices are preferred.
Wound Dressing. Polymeric matrices have been used extensively as superior wound dressing preparations. Currently, hydrogel and hydrocolloid wound dressing materials are being increasingly used due to their superior wound healing properties. For this application, a macromer system in liquid form is applied to the wound site and subsequently formed into a flexible polymeric matrix upon exposure to light. When applied as a liquid, the macromer preparation conforms to the irregular surface of the wound. Upon illumination, a flexible matrix is formed which is completely conformal to the surface of the wound; no fluid-filled pockets which can act as sites of bacterial infiltration can exist. In one embodiment, the macromer system additionally includes one or more therapeutic agents, such as growth factors or antimicrobial agents which are slowly released into the wound. Both degradable and non-degradable macromer systems can be used for this application.
In Situ Device Formation. Polymeric materials can be implanted into the body to replace or support the function of diseased or damaged tissues. One example of this is the use of hollow cylindrical polymeric devices to support the structure of a coronary artery following percutaneous transluminal coronary angioplasty (PTCA). Currently, pre-formed cylindrical devices are implanted via catheter insertion followed by balloon expansion to secure the device. The expanded device supports the structure of the artery and prevents the reversion of the artery to the closed position (restenosis).
For this application, a liquid macromer preparation could be applied to an injured artery via a multi-lumen catheter containing an illumination element. After application of the liquid macromer system to the injured tissue, a semi-rigid polymeric matrix can be formed by a brief illumination. Upon removal of the catheter, a hollow, cylindrical, conformal polymeric device remains to support the artery and prevent restenosis. In one embodiment, the macromer system additionally includes a releasable therapeutic agent or agents, such as antiproliferative and/or antithrombotic drugs. These agents are slowly released from the formed matrix, to provide additional therapeutic benefit to the injured tissues. Both degradable and non-degradable macromer systems can be used for this application.